PKC plays an important role is signal transduction. It's believed to be activiated by phosphorylation by an unknown enzyme, followed by autophosphorylation. Determination of the primary phosphorylation site(s) may lead to understanding how this important enzyme is activated and what other protein(s) may participate in the activation process. Mass spectrometry is an excellent tool for identifying the modified peptides in a digestion mixture, even if they are present in non-stoichiometric quantities. CID analysis of these species can locate the phosphate group more precisely on the peptide chain. On-line LC/MC will be used to screen the tryptic digest of PKC for phosphopeptides. Tandem mass spectrometry will be used to locate the modifications.